Formula: Hg3S2(Br,Cl,I)2
Origin of name: For Oleg Grechishchev, Russian geologist
Crystal system: Tetragonal
Crystal class: -42m
Twinning:
Unit cell:
Colour: Bright or dark orange, darkens to black
Diaphaneity:
Luster: Vitreous to adamantine
Habit: Miniute equant prims to slightly elongate grains to 0.2 mm, powdery masses and concretions of short prismatic crystals to 0.3 mm
Hardness: 2.5
Specific gravity:
Cleavage: 1; Distinct parallel to prism face
Tenacity:
Streak: Deep yellow to yellow with orange tint
Synonyms/varieties:
Comments: Dimorphous with arzakite; slowly darkens on exposure to brown-orange, then black
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